Final answer:
For a parallel-plate capacitor connected to a fixed voltage source, having the plates close together maximizes energy storage due to an increase in capacitance which leads to more charge being stored.
Step-by-step explanation:
To store the maximum amount of energy in a parallel-plate capacitor with a fixed voltage source, it is better to have the plates close together. The closer the conducting plates are, the greater the capacitance, because the attraction between the opposite charges on the plates is stronger when they are near each other. A higher capacitance for a given voltage results in more charge being stored, and therefore, more energy. The capacitance C of a parallel plate capacitor is given by the formula C = εA/d, where ε is the permittivity of the dielectric material between the plates, A is the area of the plates, and d is the separation distance between the plates. Having the plates close together (smaller d) increases C, and thus the energy stored in the capacitor, which is calculated using the formula E = 1/2 C V², where V is the voltage.